The amount of sugar and cornstarch needed will depend on the sweetness and ripeness of the fruit. Taste the berries. If they are sour, add an extra 1/4 cup sugar in step 2. If juicy, add an extra 1 teaspoon cornstarch. To catch any juices that may bubble over when baking, place the baking dish on a rimmed baking sheet or lay a piece of foil on the rack below the cobbler.